The present invention relates to method and apparatus for measuring a change in state, particularly a change in viscosity of a subject fluid utilizing the so-called hot wire method, such method and apparatus being applicable, for example, to determination of a gelation period during a gelation of foodstuff, measurement of a change in viscosity of adhesive, slurry or the like, determination of microorganism density from a change in viscosity due to proliferation of microorganism during a cultivating process, and determination of a microorganism product concentration or the like in relation to a change in viscosity thereof.
Concerning such method and apparatus for measuring a change in state of a subject fluid, various proposals have already been made.
Japanese Utility Model Laid-Open Application No. 1987-126751 discloses a method for measuring a fluid viscosity based on a velocity at which a steel ball falls in a subject fluid.
However, this dynamic method can not be used for a fluid which is susceptible to structural destruction because a steel ball will usually impose a heavy force on the fluid to be measured. In addition, such method requires sampling and therefore makes a continuous measurement impossible. Furthermore, the measurement must be done in an environment free from vibration to insure that the measurement is never affected by a vibration caused by various external factors.
Some of the inventors of the present application have previously disclosed, in Japanese Patent Laid-Open Application No. 1987-185146, a method for measuring a change in state such as viscosity of a subject fluid by measuring a change in heat transfer from a heating element immersed in the subject fluid to the latter.
Such method for measurement is most effectively useful for a static system and further more effectively useful for a state of laminar flow. But in practical use a turbulent flow often must be measured, wherein there is the necessity of providing a stable measurement environment.
To measure a subject fluid in the flowing system, there have already been proposed various methods such as the method adapted to sample a quantity of the flowing subject fluid and then to measure the fluid of the sample in a static system; the method adapted to introduce a quantity of the subject fluid into a bypass within which the actual measurement is made; and the method adapted to make a collective measurement based on outputs from one or more sensors provided within a flowing system. However, the method relying upon sampling is susceptible to generation of microorganism and fluctuations of the measurement depending on the particular location of the system at which said sampling occurs. The method employing the bypass inevitably leads to a complexity of the apparatus and it is often difficult to wash the bypass. And the method employing one or more sensors also requires an apparatus which is correspondingly complicated and sometimes expensive, depending on the number of actually employed sensors, and requires complex analysis of respective output values provided from these sensors.
Some of the inventors of the present application disclosed, in Japanese Patent Laid-Open Application No. 1988-212840, a method for measuring a change in the state of a subject fluid where a quantity of the fluid surrounding a heating sensor is maintained in a state of static flow. Thus, it is possible to measure the state of the fluid, in view of the fact that otherwise there would be generated a turbulent flow around the sensor so that the heat transfer coefficient of fluid would be changed by a change of flow velocity.
With such method of prior art, however, the sensor is susceptible to an unacceptably heavy force imposed by a piston during introduction of the fluid into a tubular body. Additionally, such technique of well known art is unsuitable for fluids of a type such that the quantity of the fluid being present externally of the tubular body tends to exhibit a state continuously changing as the time elapses, because the measurement can be made only within the tubular body.